1. Field of the Invention
The present invention relates to a cutting insert for mounting on a cutting tool body and a cutting tool.
2. Description of the Related Art
An example of a cutting insert for installation on an edge replaceable cutting tool is disclosed in Japanese Patent Laid-Open No. 2000-107911. According to the description in Japanese Patent Laid-Open No. 2000-107911, the cutting insert (throwaway chip) in Japanese Patent Laid-Open No. 2000-107911 is devised to deal with cutting fields of cutting from finished cutting to light and medium cutting. The cutting insert is generally shaped like a rhombic plate, and includes a lower surface serving as a seating face to be seated on a cutting tool body and an upper surface lying opposite the lower surface.
A nose portion is formed at an acute corner of a rake face on the upper surface, and in this nose portion a nose portion cutting edge connected to two straight cutting edges is formed. The nose portion projects from the lower surface in a direction in which the distance from the lower surface increases. The nose portion cutting edge is shaped like a protruding circular arc. Thus, rake angle varies in an increasing manner from an end point toward a central point of the nose portion cutting edge, and is largest at the central point. Furthermore, a chip breaker is formed on the upper surface and is formed of a top surface that is a plane substantially parallel to the lower surface and a breaker wall surface raised from the rake face to the top surface in an inclined manner.
In the cutting insert described in Japanese Patent Laid-Open No. 2000-107911, the rake face along cutting edges located away from the opposite ends of the nose portion cutting edge have a small rake angle. Thus, in cutting using the cutting insert, chips generated at these cutting edges are generally thick. Therefore, the chips and the cutting edges become hot, and the cutting insert is increasingly worn away and may have a shortened life.
Moreover, according to the description in Japanese Patent Laid-Open No. 2000-107911, chips generated at the cutting edges such as the nose portion cutting edge travel over the surface of the cutting insert while slipping over the rake face and the nose portion rake face, and collide against the breaker wall surface of the chip breaker. Hence, particularly if the chips generated are thick, the rake face is more likely to be worn away. Therefore, such a cutting insert is problematic in terms of tool life.